1. Field of the Invention
The present invention relates to an S/N ratio improvement technology of a reproduced signal in an optical disc reproduction apparatus.
2. Description of the Related Art
The recording capacity of an optical disc has been improved from the Compact Disc (CD) up to the Blu-ray Disc™ (BD) through the Digital Versatile Disc (DVD) due to miniaturization of the optical spot achieved by decrease in the wavelength of the light source and increase in the numerical aperture of the objective lens. However, since the optical spot size has nearly reached a limit in the BD using the blue light source and the high numerical aperture objective lens, a multilayer structure for increasing the number of recording layers for recording the information is one of the dominant measures to further increase the recording capacity. When realizing the multilayer recording layer, since the larger the number of recording layers becomes, the lower the intensity of the reflected light returning to the optical pickup becomes, the influence of the detection system noise such as amplifier noise increases, and the signal-to-noise ratio (S/N ratio) of the reproduced signal drops. Further, due to increase in the recording capacity, increase in the data transfer rate is also required. When increasing the data transfer rate, due to increase in the detection system noise caused by broadening the bandwidth of the reproduced signal, the S/N ratio of the reproduced signal drops. As described above, since the multilayer structure and the increase in the data transfer rate cause the drop of the S/N ratio, it is obvious that it becomes difficult to keep the reliability of the data with the reproduction method in the related art. Therefore, an S/N ratio improvement technology of the reproduced signal becomes important for the future improvement of the performance of optical discs.
As the S/N ratio improvement technology of the reproduced signal in the optical disc, an optical information detection method using Homodyne phase diversity detection is described in U.S. Pat. No. 7,750,276 (Document 1), for example. Specifically, by making the signal light reflected by the optical disc and the reference light branched from the same light source and guided to the detector without being reflected by the optical disc interfere with each other on the detector, an amplified reproduced signal is obtained. On this occasion, by simultaneously obtaining the detector outputs in four types of interference states with the phase relationships between the reference light and the signal light shifted 90 degrees from each other and then performing calculation, a stable reproduced signal can be obtained without being affected by the light path length variation due to flapping of the disc. According to this method, since the amplitude of the reproduced signal is amplified without increasing the amplifier noise, the S/N ratio is improved. Further, in JP-A-2010-170616 (Document 2), there is described an optical information reproduction method of monitoring the calculation process, the calculation output, or both of the calculation process and the calculation output in the Homodyne phase diversity detection, and disposing a calculation control mechanism for controlling the calculation so that the output is stabilized to thereby avoid the influence of the variation of the constituent components, the aging variation of the characteristics, and so on, and stably obtain the amplification effect.
Here, a related art detection method (hereinafter referred to as a normal detection method) not using the Homodyne phase diversity detection will be explained. The light emitted from a laser diode of a light source is collected by an objective lens, and the optical disc is irradiated with the light. The reflected light (hereinafter referred to as a signal light) from the optical disc is detected by a detector, and a reproduced signal is generated. The reproduced signal is sampled by an analog/digital converter (A/D converter) to thereby be digitalized, and the subsequent process is performed using digital calculation. An equalization process, a demodulation process, and a decoding process are performed on the reproduced signal thus digitalized to thereby restore user data.
There are several conversion methods used in the A/D converters, and FIG. 14 shows a configuration of a flash type as an example. In this method, an input voltage and voltage values obtained by dividing the reference voltage with resistors are compared with each other using 2n-1 voltage comparators, and the result is encoded into a binary number using an encoder, and thus, an n-bit digital value can be obtained. The number n of bits of the digital value represents the vertical resolution of the A/D converter. In the case in which the vertical resolution is n bits, the number of signal levels after the digitalization is 2n.
In the A/D converter, the quantization noise is caused by the error due to the conversion of an analog value into a digital value. The influence thereof becomes nonnegligible in the case in which the vertical resolution decreases, and thus, the S/N ratio of the reproduced signal is deteriorated. However, since the improvement of the vertical resolution is associated with increase in circuit size of the A/D converter and decrease in operation speed thereof, the vertical resolution is determined to be a level necessary and sufficient to make the influence of the quantization noise negligible. In the optical disc reproduction apparatus of the normal detection type of the related art, the vertical resolution of the A/D converter is typically determined to be 7 through 8 bits.
In contrast, in the case of the optical disc reproduction apparatus using the Homodyne phase diversity detection described in Document 1, the output signals IPD1, IPD2, IPD3, and IPD4 of four photodetectors PD1, PD2, PD3, and PD4 corresponding to the four types of interference states having the phase relationships between the signal light and the reference light 90 degrees different from each other are as follows.
                              I                      PD            ⁢                                                  ⁢            1                          =                                                                                                                1                    2                                    ⁢                                      E                    sig                                                  +                                                      1                    2                                    ⁢                                      E                    ref                                                                                      2                    =                                                    1                4                            ⁢                                                                                      E                    sig                                                                    2                                      +                                          1                4                            ⁢                                                                                      E                    ref                                                                    2                                      +                                          1                2                            ⁢                                                                E                  sig                                                            ⁢                                                                E                  ref                                                            ⁢                              cos                ⁡                                  (                                                            φ                      sig                                        -                                          φ                      ref                                                        )                                                                                        (        1        )                                          I                      PD            ⁢                                                  ⁢            2                          =                                                                                                                1                    2                                    ⁢                                      E                    sig                                                  -                                                      1                    2                                    ⁢                                      E                    ref                                                                                      2                    =                                                    1                4                            ⁢                                                                                      E                    sig                                                                    2                                      +                                          1                4                            ⁢                                                                                      E                    ref                                                                    2                                      -                                          1                2                            ⁢                                                                E                  sig                                                            ⁢                                                                E                  ref                                                            ⁢                              cos                ⁡                                  (                                                            φ                      sig                                        -                                          φ                      ref                                                        )                                                                                        (        2        )                                          I                      PD            ⁢                                                  ⁢            3                          =                                            1              8                        ⁢                                                                                                                      (                                              1                        -                        ⅈ                                            )                                        ⁢                                          E                      sig                                                        +                                                            (                                              1                        +                        ⅈ                                            )                                        ⁢                                          E                      ref                                                                                                  2                                =                                                    1                4                            ⁢                                                                                      E                    sig                                                                    2                                      +                                          1                4                            ⁢                                                                                      E                    ref                                                                    2                                      +                                          1                2                            ⁢                                                                E                  sig                                                            ⁢                                                                E                  ref                                                            ⁢                              sin                ⁡                                  (                                                            φ                      sig                                        -                                          φ                      ref                                                        )                                                                                        (        3        )                                          I                      PD            ⁢                                                  ⁢            4                          =                                            1              8                        ⁢                                                                                                                      (                                              1                        +                        ⅈ                                            )                                        ⁢                                          E                      sig                                                        +                                                            (                                              1                        -                        ⅈ                                            )                                        ⁢                                          E                      ref                                                                                                  2                                =                                                    1                4                            ⁢                                                                                      E                    sig                                                                    2                                      +                                          1                4                            ⁢                                                                                      E                    ref                                                                    2                                      -                                          1                2                            ⁢                                                                E                  sig                                                            ⁢                                                                E                  ref                                                            ⁢                              sin                ⁡                                  (                                                            φ                      sig                                        -                                          φ                      ref                                                        )                                                                                        (        4        )            
Here, Esig denotes the amplitude of the signal light, Eref denotes the amplitude of the reference light, φsig denotes the phase of the signal light, and φref denotes the phase of the reference light. For the sake of simplicity, it is assumed that Esig and Eref are perfectly coherent with each other. Differential signals are obtained for each of the pairs of the signals IPD1 and IPD2, and IPD3 and IPD4, and the two differential signals Sig1 and Sig2 are expressed as follows.Sig1=IPD1−IPD2=|Esig∥Eref| cos(φsig−φref)  (5)Sig2=IPD3−IPD4=|Esig∥Eref| sin(φsig−φref)  (6)
According to the formulas (5) and (6), Sig1 and Sig2 each have the envelope of the waveform varying in accordance with the variation of the phase difference (φsig−φref) between the signal light and the reference light. The variation of the phase difference is caused by the variation of the light path length of the signal light due to, for example, flapping of the disc.
Sig1 and Sig2 are respectively digitalized by the A/D converters, and then added to each other after being respectively squared to thereby form the reproduced signal S, which is proportional to the intensity |Esig|2 of the signal light independently of the phase difference (φsig−φref), as expressed in the formula (7). Subsequently, by substantially the same process as in the case of the normal detection method, the recorded data is restored.S=(Sig1)2+(Sig2)2=|Esig|2|Eref|2  (7)
Hereinafter, a problem in putting the optical disc reproduction apparatus using the Homodyne phase diversity detection into practical use will be described.
FIG. 1 is a graph obtained by calculating and then plotting the waveform of the reproduced signal in the normal detection using a diffractive optical simulator. The horizontal axis represents time (the unit is a channel bit period T), and the vertical axis represents signal intensity (arbitrary unit). As the calculation condition, the wavelength of the light source is set to 405 nm, and the numerical aperture (NA) of the objective lens is set to 0.85. It is assumed that a random pattern is used as the recorded data, and the channel bit length is set to 74.5 nm. It should be noted that any noise such as the medium noise or the amplifier noise is not added at all to the signal. The reference symbols IH and IL shown in the drawing respectively indicate an upper envelope level and a lower envelope level of the reproduced signal. The modulation m of the reproduced signal is defined as a value obtained by dividing the total amplitude of the reproduced signal by the upper envelope level, namely m=(IH−IL)/IH, and m=0.4 is obtained in the waveform shown in the drawing.
FIG. 3A is a graph obtained by calculating and then plotting the relationship between the vertical resolution and the jitter of the A/D converter with respect to the reproduced signal shown in FIG. 1. This graph is obtained by digitalizing the waveform of the reproduced signal shown in FIG. 1 at a variety of levels of the vertical resolution, then performing, for example, the equalization process on the waveform, and then measuring the Data-to-Clock jitter. According to FIG. 3A, the lower the vertical resolution becomes, the higher the level of the jitter becomes, which shows the fact that the influence of the quantization noise is increased due to the insufficient vertical resolution. FIG. 3B is a graph obtained by plotting the relationship between the vertical resolution and the increment of the jitter due to the quantization noise, the relationship being obtained from the result of FIG. 3A. Here, the increment Δσ of the jitter due to the quantization noise can be calculated as Δσ=√(σ2−σ02) denoting the jitter value at each of the vertical resolution levels with σ, and the jitter value at a sufficiently high vertical resolution level (8 bits here) with σ0. As a rough criteria of the level of the jitter increment, if Δσ exceeds about 3%, the influence of the quantization noise becomes nonnegligible on an empirical basis. Therefore, according to FIG. 3B, in the case of the normal detection method of the related art, if the vertical resolution becomes equal to or lower than 5 bits, the S/N ratio of the reproduced signal is deteriorated due to the influence of the quantization noise. In the actual reproduction apparatus, the vertical resolution is generally set to 7 or 8 bits taking the amplitude margin of the input signal to the A/D converter into consideration.
Then, FIG. 2 shows the waveforms of the differential signals Sig1 and Sig2 in the case of performing the Homodyne phase diversity detection in the condition in which the waveform of the reproduced signal in the normal detection shown in FIG. 1 is obtained. These waveforms are calculated using the formulas (5) and (6) assuming that the reproduced signal due to the normal detection is |Esig|2. It is assumed that the intensity |Eref|2 of the reference light is 100 times as high as the intensity |Esig|2 of the signal light. In the calculation, the disc flapping is virtually provided by varying the phase difference (φsig−φref) between the signal light and the reference light in a range of 0 through 2π at a constant variation (period of about 14,000 T). According to FIG. 2, in Sig1 and Sig2, the envelopes of the waveform vary in accordance with the variation of the phase difference (φsig−φref). The outer envelope and the inner envelope of the waveform thus varying correspond respectively to the upper envelope and the lower envelope of the reproduced signal due to the normal detection. In other words, the amplitude of the reproduced signal component in the differential signal corresponds to the thickness of the curve representing the differential signal waveform.
FIG. 17A is a graph showing the relationship between the vertical resolution and the jitter of the A/D converter in the case of performing the Homodyne phase diversity detection on the two differential signals shown in FIG. 2. This graph is obtained by digitalizing the two differential signals shown in FIG. 2 at a variety of levels of the vertical resolution, generating the reproduced signal by performing square sum calculation on the two differential signals thus digitalized, then performing, for example, the equalization process thereon, and then measuring the Data-to-Clock jitter. According to FIG. 17A, in the case of the Homodyne phase diversity detection, although the tendency that the lower the vertical resolution becomes, the more the jitter increases is substantially the same as in the case of the normal detection method shown in FIG. 3A, the jitter starts increasing at a higher vertical resolution than in the case of the normal detection. FIG. 17B is a graph obtained by plotting the relationship between the vertical resolution and the increment of the jitter due to the quantization noise, the relationship being obtained from the result of FIG. 17A. According to the result of FIG. 17B, if substantially the same rough criteria as in the case of the normal detection method described above is applied with respect to the level of the jitter increment, the S/N ratio of the reproduced signal is deteriorated due to the influence of the quantization noise at the vertical resolution equal to or lower than 8 bits in the case of the Homodyne phase diversity detection. As described above, in the Homodyne phase diversity detection method, the vertical resolution at which the S/N ratio starts being affected by the quantization noise rises to a level higher than in the case of the normal detection method. The reason therefor will be explained below.
In the Homodyne phase diversity detection described in Document 1 and Document 2, the two differential signals are digitalized by the A/D converter, and the subsequent squaring calculation and adding calculation are performed by digital signal processing. When digitalizing a signal by the A/D converter, in general, the amplitude of the input signal is previously adjusted to be approximated to the full-scale input range of the A/D converter to thereby make full and effective use of the vertical resolution. For example, in the case of the reproduced signal due to the normal detection method shown in FIG. 1, the total amplitude (IH−IL) is approximated to the full-scale input range of the A/D converter.
In contrast, in the case of the Homodyne phase diversity detection, the total amplitude Adif of each of the differential signals Sig1 and Sig2 shown in FIG. 2 is approximated to the full-scale input range of the A/D converter, and then the signals are input. However, since the amplitude aRF of the reproduced signal component in each of the difference signals Sig1 and Sig2 is significantly small with respect to the total amplitude Adif as shown in FIG. 2, the effective vertical resolution with respect to the reproduced signal component is remarkably small compared to the case of the normal detection. Therefore, if the vertical resolution of the A/D converter is not sufficiently high, the S/N ratio of the differential signal after being digitalized is deteriorated due to the influence of the quantization noise, and thus, the S/N ratio of the reproduced signal generated from the differential signals is also deteriorated. The reason that, in the Homodyne phase diversity detection method, the vertical resolution, at which the S/N ratio starts being affected by the quantization noise, rises to a level higher than in the case of the normal detection method is as described above.
In the case of the Homodyne phase diversity detection method, the effective vertical resolution with respect to the reproduced signal component aRF in the differential signal is decreased to a level (aRF/Adif) times of that of the case of the normal detection method. The value (aRF/Adif) varies within the range shown in the formula (8) in accordance with the variation of aRF due to the variation of the phase difference.
                    0        ≤                              a            RF                                A            dif                          ≤                              1            -                                          1                -                m                                              2                                    (        8        )                                m        =                                            I              H                        -                          I              L                                            I            H                                              (        9        )            
Here, m denotes the modulation of the reproduced signal in the case of the normal detection.
In each of the signals Sig1 and Sig2, (aRF/Adif) varies in accordance with the phase difference (φsig−φref), the effective vertical resolution varies, and the significance of the influence of the quantization noise also varies in accordance with the phase difference. However, regarding the reproduced signal S generated by the square sum calculation of Sig1 and Sig2, the influence of the quantization noise is maximized in the case in which the amplitude of the reproduced signal component in Sig1 and the amplitude of the reproduced signal component in Sig2 are equal to each other. The case corresponds to the case in which the phase difference takes (2n+1)π/4 wherein n denotes an integer, and in this case, (aRF/Adif) is expressed as the formula (10).
                                          a            RF                                A            dif                          =                              1            -                                          1                -                m                                                          2            ⁢                          2                                                          (        10        )            
Therefore, the proportion of the effective vertical resolution with respect to the amplitude a of the reproduced signal component in the differential signal to that of the reproduced signal due to the normal detection is as follows.
      1    -                  1        -        m                  2    ⁢          2      
Therefore, the loss ΔR (bits) of the vertical resolution is expressed as the formula (11) at worst.
                              Δ          ⁢                                          ⁢          R                =                              log            2                    (                                    2              ⁢                              2                                                    1              -                                                1                  -                  m                                                              )                                    (        11        )            
For example, in the case in which the value of the modulation m is 0.4, which is defined as the lower limit in the BD standard, the loss ΔR of the vertical resolution is about 3.6 bit. Therefore, even if the A/D converter with the vertical resolution of 8 bit is used, the vertical resolution of about 4.4 bit can only be obtained effectively. Therefore, the S/N ratio of the reproduced signal is deteriorated due to the influence of the quantization noise, and it becomes unachievable to maximally obtain the amplification effect of the signal amplitude due to the Homodyne detection.